Heating of tissues can be used as a form of cancer treatment. Such heating, known as hyperthermia therapy, is generally used as a method to treat cancer, either by just its thermal effects or in combination with other agents, e.g. radiation or pharmaceutical cancer treatments. That is, this heating process is used to kill cancer cells and/or damage cancer cells, making such cells more sensitive to collateral modes of treatment. In general, such therapies require precise control of temperature, as increasing the temperature of tissue to 50 Celsius (“C”) can cause coagulation necrosis (i.e., ablation). Typically, such therapies are carried using different forms of energy, including microwaves, radio waves and ultrasound.
In general, the irradiation time for hyperthermia depends on the maximum temperature the target can reach over an extended time, which could be on the order of hours. Although the temperatures used for treatment generally do not exceed 42 C, some treatments near 50 C have been performed. Such higher temperatures are of interest since increasing the temperature allows exposure time to be reduced. For example, a hyperthermia treatment can be performed by increasing the local body temperature to about 50 C. However, such therapies are still reported to require exposure times on the order of 0.1 hours (6 minutes). [See, e.g., Dickson, J. A. and Calderwood, S. K., 1980, “Temperature Range and Selective Sensitivity of Tumors to Hyperthermia: A Critical Review,” Annals New York Academy of Sciences, Vol. 335, pp. 180-205].
In treating tumors or other cancers, it is desirable that the tumor be locally heated such that healthy tissue remains unaffected. Such a treatment can be affected through external approaches, such as that undertaken for treating skin cancer. An external approach to treating skin cancer, for example, utilizes high frequency microwaves with slight tissue penetration depth to induce cell death in melanoma tumors. For treating deeper lying tumors, however, existing methods generally rely on the insertion of heat sources directly into a tumor to induce cell death or to cause ablation. To overcome the drawbacks associated with such an approach, there has been a move in the field to focus electromagnetic radiation in deeper lying tissues to reach the tumor. Care must be taken, however, to limit the diffusion of heat throughout healthy tissue that surrounds the tumor.